How to replace an orphaned external file by an existing one? (all instances)
You can set the filepath attribute on your sequences with python:

import bpy
for seq in bpy.data.scenes['Scene'].sequence_editor.sequences_all:
    if 'image_a' in seq.elements[0].filename:
        seq.elements[0].filename = "image_b.png"


You can optionally change the files location setting seq.directory if needed.